Its really not that complicated. The gist of the situation is closer to a 'flame war' than you think.
Apple is flush with cash. They probably decided to sue to:
1. At worst, slow their multiple competitors down
2. At best get injunctions against competing devices or features.
Some people might claim legitimate infringement, but it is hard to take such claims seriously when there is so much prior art and so many overlapping patents for obvious methods in the wild.
Large companies respond to lawsuits by starting their own lawsuits, which largely devolve into he-said/she-said arguments. Sometimes the companies can settle it nicely (cross patent licensing agreements), sometimes compromises are made (courts force 'fairly priced' licensing for critical patents), and sometimes one side just wins the case.
It is important to note who apple has and hasn't sued. Sony makes tablets and Android phones, but Apple probably doesn't want to jeapordize iTunes access to Sony's music library by antagonizing their parent company.
HTC's lawsuit is simply self defense. There is no reason to not fight back when you're being picked on by a bully who has already decided to beat you up.
Stay tuned for Apple to claim collusion against them when all the companies they have picked fights with (Samsung, Motorola) decide to fight back.
Many people won't want to admit it, but deep down, the reason Apple is suing now is because they completely understand that they cannot hope to perpetually stay ahead of 4 or 5 of the biggest electronics manufacturers in the world who only have to do half the work to put out a tablet (Google is supplying them with software for free).
Either the quality of the product or their profit margins will take a hit in the long run. Apple is just doing what it can as soon as it can to maintain their current position.
